Preheat oven to 180 degrees C. In a small bowl, pour the rum over the raisins and set aside. Grease a 23cm baking pan.
In a medium bowl, cream the sugar and butter. Add the egg and mix until fluffy. Sift together the flour and baking powder. Add this alternately to the egg mixture with the milk. Mix until blended, then fold in raisins.
Spread the batter evenly in the greased pan and bake in preheated oven for 20 to 25 minutes.
While the raisin bars are baking, in a smaller bowl, stir together the 2 tablespoons of butter with the icing sugar. Add the remaining rum and stir until smooth.
When the bars come out of the oven, let them cool slightly, then spread the icing evenly over the top. Cut into bars.